/**
 * @fileoverview Rule to flag `else` after a `return` in `if`
 * @author Ian Christian Myers
 */

"use strict";

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------

module.exports = {
    meta: {
        docs: {
            description: "disallow `else` blocks after `return` statements in `if` statements",
            category: "Best Practices",
            recommended: false
        },

        schema: []
    },

    create(context) {

        //--------------------------------------------------------------------------
        // Helpers
        //--------------------------------------------------------------------------

        /**
         * Display the context report if rule is violated
         *
         * @param {Node} node The 'else' node
         * @returns {void}
         */
        function displayReport(node) {
            context.report({ node, message: "Unnecessary 'else' after 'return'." });
        }

        /**
         * Check to see if the node is a ReturnStatement
         *
         * @param {Node} node The node being evaluated
         * @returns {boolean} True if node is a return
         */
        function checkForReturn(node) {
            return node.type === "ReturnStatement";
        }

        /**
         * Naive return checking, does not iterate through the whole
         * BlockStatement because we make the assumption that the ReturnStatement
         * will be the last node in the body of the BlockStatement.
         *
         * @param {Node} node The consequent/alternate node
         * @returns {boolean} True if it has a return
         */
        function naiveHasReturn(node) {
            if (node.type === "BlockStatement") {
                const body = node.body,
                    lastChildNode = body[body.length - 1];

                return lastChildNode && checkForReturn(lastChildNode);
            }
            return checkForReturn(node);
        }

        /**
         * Check to see if the node is valid for evaluation,
         * meaning it has an else and not an else-if
         *
         * @param {Node} node The node being evaluated
         * @returns {boolean} True if the node is valid
         */
        function hasElse(node) {
            return node.alternate && node.consequent && node.alternate.type !== "IfStatement";
        }

        /**
         * If the consequent is an IfStatement, check to see if it has an else
         * and both its consequent and alternate path return, meaning this is
         * a nested case of rule violation.  If-Else not considered currently.
         *
         * @param {Node} node The consequent node
         * @returns {boolean} True if this is a nested rule violation
         */
        function checkForIf(node) {
            return node.type === "IfStatement" && hasElse(node) &&
                naiveHasReturn(node.alternate) && naiveHasReturn(node.consequent);
        }

        /**
         * Check the consequent/body node to make sure it is not
         * a ReturnStatement or an IfStatement that returns on both
         * code paths.
         *
         * @param {Node} node The consequent or body node
         * @param {Node} alternate The alternate node
         * @returns {boolean} `true` if it is a Return/If node that always returns.
         */
        function checkForReturnOrIf(node) {
            return checkForReturn(node) || checkForIf(node);
        }


        /**
         * Check whether a node returns in every codepath.
         * @param {Node} node The node to be checked
         * @returns {boolean} `true` if it returns on every codepath.
         */
        function alwaysReturns(node) {
            if (node.type === "BlockStatement") {

                // If we have a BlockStatement, check each consequent body node.
                return node.body.some(checkForReturnOrIf);
            }

            /*
             * If not a block statement, make sure the consequent isn't a
             * ReturnStatement or an IfStatement with returns on both paths.
             */
            return checkForReturnOrIf(node);
        }

        //--------------------------------------------------------------------------
        // Public API
        //--------------------------------------------------------------------------

        return {

            IfStatement(node) {
                const parent = context.getAncestors().pop();
                let consequents,
                    alternate;

                // Only "top-level" if statements are checked, meaning the first `if`
                // in a `if-else-if-...` chain.
                if (parent.type === "IfStatement" && parent.alternate === node) {
                    return;
                }

                for (consequents = []; node.type === "IfStatement"; node = node.alternate) {
                    if (!node.alternate) {
                        return;
                    }
                    consequents.push(node.consequent);
                    alternate = node.alternate;
                }

                if (consequents.every(alwaysReturns)) {
                    displayReport(alternate);
                }
            }

        };

    }
};
